In existing printing systems, bitmapped data are sometimes obtained from fairly low resolution image generating sources. One type of source, low resolution scanners, of which there is already a large infrastructure, converts information on a scanned page to sampled image data, but then frequently does other processing on the image, such as halftoning or thresholding, which reduces information content and produces a low resolution bitmap image. Another kind of source, image generators, may also produce a low resolution bitmap from its respective source information. These low resolution bitmaps are generally intended to be printed on low resolution printers.
These low resolution bitmaps can be characterized as having artifacts such as jaggies (visible stairstepping in almost horizontal or almost vertical edges, for example), and are also frequently device dependent, being generated according to the characteristics of a particular printer and sent directly to that printer's photoreceptor. For example, the image may be processed to be printed on a write-black or write-white bitmap printer at a particular resolution.
Once the image is converted to a bitmap representation the loss of fidelity due to the coarseness of the bitmap is generally irreversible. However, the stairstepping artifact, for example, has proven to be identifiable to a limited extent, and therefore somewhat removable in a high addressability printer.
High addressability display or printing systems expect to utilize high resolution bitmapped or numerical data representing an image, such as provided by a high resolution scanner or image generator.
In order to print low resolution bitmaps on a high addressability printer, the images must be processed in order to enhance the fidelity and increase the density of the low resolution bitmaps. A limited amount of fidelity may be restored to the low resolution image using a bitmap image enhancement technique such as template matching. Such enhancement techniques generally produce an enhanced resolution bitmap output by providing an enhanced bit or set of bits for each bit of the input image. These bit patterns are also typically dependent upon the characteristics of the photoreceptor of the printer that the enhanced image is developed for.
The present invention provides a method for enhancing the fidelity of these low resolution bitmapped images for printing on high-addressability printing systems by providing a multiple-bit-per-pixel sampled image output similar to that produced by a scanner which has not converted the image into a bitmap. Because this enhancement data is numerical, it is not dependent upon the characteristics of any particular system on which it will be printed. Provided to the high-addressability printer is enhancement data which comprises a device independent, numerical output sample for each pixel location at the resolution of the original bitmap image. Armed with that value, the printer intelligence may render that pixel in accordance with the printer's unique characteristics.
Microaddressable printers and other types of high addressability display systems can operate in an "overscanned" mode to render images by scanning one or more intensity modulated scan spots over a photosensitive recording medium in accordance with a scan pattern that causes the spot or spots to superimpose multiple discrete exposures on the recording medium on centers that are separated by a pitch distance that is significantly less than the effective spatial diameter of the scan spot (i.e., the full width/half maximum (FWHM) diameter of a gaussian scan spot). The technique of microaddressability via overscanned illumination is more fully described in the copending, coassigned U.S. Pat. No.5,138,339 of D. N. Curry et al., entitled "MICROADDRESSABILITY VIA OVERSCANNED ILLUMINATION FOR OPTICAL PRINTERS AND THE LIKE HAVING HIGH GAMMA PHOTOSENSITIVE RECORDING MEDIA", incorporated herein by reference.
For high quality printing, high-addressability printing systems, whether they are overscanned or not, require a high fidelity input source, such as sampled image data or a high resolution bitmap. A typical high resolution scanner is capable of producing high resolution, grayscale sampled image data at 800 spots per inch (spi), for instance, with each spot of picture element ("pixel") represented by multiple bits. However, a large number of existing image scanners produce low resolution bitmapped image data at 300 or 400 spi with one bit per pixel, which is not enough resolution or fidelity to eliminate unwanted artifacts induced by the coarseness of the information, such as stairstepping on angled contours.
It is an object of the present invention to provide device independent sampled multiple-bit-per-pixel numerical output for a high-addressability printer from low resolution bitmap images, such as those produced by the current infrastructure of existing scanners or image generators. In this way, existing low resolution bitmapped image generators would be able to drive high-addressability printers that process sampled data, and existing low resolution bitmap images would be printable on high-addressability printers.
As discussed above, image enhancement methods such as template matching and gradient mask convolution techniques have been proposed to enhance images and increase resolution by more precisely controlling the size, positioning, and number of picture elements representing each enhanced pixel that are printed on a xerographic photoreceptor to render bitmapped images. However, all have the characteristic that they generally produce pulse width modulated data streams representing pixel patterns for a laser diode in the fast scan direction.
For example, Walsh et al., U.S. Pat. No. 4,437,122, describes a method of enhancing the resolution and quality of image elements of a system receiving video display pixel information by matching the input data to predetermined templates and providing enhanced binary output pixels. Tung, U.S. Pat. No. 4,847,641, describes a technique for enhancing the printing of bitmapped images by piecewise matching of the bitmap with predetermined stored templates of patterns to detect the occurrence of preselected bitmap features. A plurality of compensation subcells are generated for the printer in response to signals indicating matching bit patterns.
Another fidelity enhancement technique, described in copending, coassigned U.S Pat. No. 5,329,599 of D. N. Curry et al., entitled "ENHANCED FIDELITY REPRODUCTION OF IMAGES BY HIERARCHICAL TEMPLATE MATCHING," incorporated herein by reference, provides enhancement for a 2.times. overscanned microaddressable printer, involving template matching and production of enhanced pixels in two dimensions.
Lung, U.S. Pat. No. 5,029,108, describes a gradient mask convolution technique for identifying pixels to be adjusted for enhancement, but once again uses this information to produce a device dependent, specific code, output.
Template matching and gradient mask convolution techniques effectively overcome some of the sampling errors that are caused by the use of input data that are too coarse to avoid stairstepping or jaggies in the image. However, techniques such as described above typically alter the resolution and produce an enhanced pixel bitmap output which is dependent upon the characteristics of the printer that it is being sent to. For each printer family, for example, a write-white or write-black family, a separate set of enhanced pixels must be developed which is unique for that family.
The present invention, however, after identifying a pixel to be enhanced, uses the information to produce a single device-independent, multiple-bit binary number representing the pixel. By nature this binary number cannot directly drive the output of a binary display or printer. On the contrary, this technique produces a multiple-bit-per-pixel sampled output, similar to sampled data an input scanner would produce. The sampled data requires further processing before it may be printed on a printer; i.e., it is device independent gray data, which can be resolution converted, eroded, dilated, thresholded, or halftoned. By storing the sampled representation instead of the bitmapped representation inside the image generator, fidelity and machine independence are maintained. The conversion to bitmap should be executed by the printer, closer to the photoreceptor, where the high capacity bitmap resulting from the conversion is stored only on the photoreceptor. It is an object of the present invention to provide printer independent image enhancement data in the form of numerical sample output data, which may be independently received for processing by a variety of high-addressability printers that can process sampled data.
Another aspect of the invention is based on the recognition that existing bitmap enhancement methods which produce output patterns are inherently inefficient. Many essentially redundant patterns are produced, so that the resolution of enhancements for n bits per pixel is (n+1) levels. Gray data is logarithmically encoded, producing 2.sup.n levels for n bits per pixel. It therefore is a further object of this invention to eliminate redundancy in the enhancement information by providing pixel enhancement information as numerical sample data, thereby increasing the edge position precision of enhancements to 2.sup.n for n bits per pixel.